Ethanol production may have a significant impact on the environment through consumption of natural resources including the raw material feedstock (corn, beans, sugar cane, cellulose, etc.), energy, water, and nutrients; production of concentrated liquids; and CO2 emission. In general, ethanol production consists of feedstock preparation; liquefaction which dissolves the feedstock into solution; enzyme conversion to sugar; fermentation of the sugar into ethanol; separation of ethanol from the distillers wet grains or “whole stillage” (residual liquids and solids); and subsequent management of the whole stillage. A significant portion of the ethanol production energy and resources is spent on the management of the whole stillage.
For example, conventional corn to ethanol production (FIG. 1) involves wet or dry milling of the corn consisting of starch (60-70%), cellulose compounds (8-10%), proteins (7-9%), oil (3-4%), ash (1-2%), and water (15%). Liquefaction of the milled corn produces a suspension called corn mash. This first enzyme step involves raising the pH to 6-7, adding ammonia and urea, and adding α-amylase enzyme and heat (first 212° F.+ followed by 175-195° F.). Saccarification of the corn mash converts the starch into sugar. During the second enzyme step, the pH is lowered to 3-4 with sulfuric acid, and glucoamylase enzyme is added to convert the starch to sugar. The sugar is then converted to ethanol through fermentation with yeast (now the solution is referred to as beer). The beer is then processed through a distillation column where the ethanol is separated from the majority of the solids and water which are removed from the column as whole stillage. Distillation produces a solution which is an azeotrope of ethanol and water (95% ethanol). The azeotropic solution is then run through mole sieves to separate the ethanol from the water.
Whole stillage, in most if not all cases, is run through a solid liquid separation system (for example, a centrifuge(s)) to separate the solids (distillers' grains) from the liquid (thin stillage). Distillers' grains have feed value and are commonly dried and sold as distillers dried grains (DDG).
The thin stillage consists of remaining dissolved and suspended solids (total solids typically 5-9%) with a pH 3-4. A portion of the thin stillage typically is returned to the front of the ethanol process (backset). Backset volumes vary by plant from 20% up to 50% with periodic increases to manage accumulation of solids. The thin stillage balance conventionally is run through evaporators to produce condensed distillers' solubles or syrup.
Syrup is managed separately or is put on the DDG and sold as distillers' dried grains with solubles (DDGS). Syrup has constituents that limit the market such as fats that compromise feed quality and degradable material that generally limit the market to within 100 miles of production. The syrup market has become increasingly competitive as ethanol production (DDG and syrup supply) increases, while demand remains relatively static. Market prices have declined 2004-2007 from approximately $17-19 per ton for condensed distillers' solubles to approximately $6 per ton in Iowa.
The energy to evaporate thin stillage to syrup is substantial. Based on thermodynamics (assuming characteristics similar to water), it requires approximately 5.4 mmBTU to dry every 1,000 gallons of thin stillage from 87% moisture to 78% moisture at a 35% heat transfer efficiency. A conventional ethanol plant that produces 40-50 million gallons per year will produce approximately 575,000 gallons per day at 30% backset. The energy cost to evaporate and dry the thin stillage is equivalent to $31,000 per day in natural gas (at $10/mmBTU) and results in 66,000 tons of CO2. Ethanol plants are designed to recover as much of the heat as reasonable, however it is nonetheless a net cost to the facility that is increasing with falling syrup market price. As a result ethanol producers have implemented alternative methods to manage the thin stillage.
Alternate methods that are available in the market include thin stillage treatment to produce water quality that can be returned to the process and thin stillage evaporation and burning to capture the BTU value of the syrup. Thin stillage treatment has consisted of anaerobic digestion with nutrient removal (if required) followed by a secondary treatment of aerobic and/or membrane (reverse osmosis or nano-filtration) treatment. The anaerobic treatment process produces biogas that can be used by the facility boilers to offset natural gas purchases. The secondary treatment with aerobic and/or reverse osmosis or nano-filtration membrane separation produces water that can be returned to the process. This secondary treatment consumes substantial energy (greater than $1,000,000 annually) in the form of electrical horsepower required to power the blowers to aerate the aerobic process and oxidize the organic and nitrogen constituents and achieve the pressure (greater than 4 bar) across the membrane. In addition, waste concentrated liquids and solids, typically 250,000 gallons±50,000 gallons per day, are produced that require off-site disposal. Commonly the operating cost of these treatment systems is relatively equivalent to the energy value recovered in the biogas.
Burning the thin stillage is another alternative that has been used to capture the energy value of the organic constituents of the thin stillage. It has been reported that the solids content has to be 12.5% or greater for the heating value to be sufficient to break even with the energy required to heat and evaporate the water present. Assuming the thin stillage has a total solids content of 6-7%, one-half (50%) of the water has to be evaporated to achieve break even energy on the burner. Based on the 40-50 million gallons a year plant example, it will cost $23,000 dollars a day in natural gas and 49,000 tons of CO2 per year to burn all of the thin stillage produced.